Dr. Volker,
The c-client-2007f-1 package is missing a header:
In file included from /usr/include/c-client/c-client.h:41:0,
from conftest.c:360:
/usr/include/c-client/mail.h:1506:21: fatal error: linkage.h: No such
file or directory
This is necessary to build PHP's IMAP extension OOTB. I have worked
around it for now, but for others to be able to rebuild this from
source, it should be fixed in the package instead.